Rhizobium leguminosarum is a well-known example of a symbiotic bacterium that forms a mutualistic relationship with leguminous plants (such as peas, beans, and lentils). In this association, the bacteria live inside specialized structures called root nodules. The relationship is mutually beneficial because:

The Bacteria: Receive nutrients (carbohydrates and organic acids) and a protected environment from the plant.

The Plant: Receives converted atmospheric nitrogen (ammonia), which it uses for growth and development—a process known as biological nitrogen fixation.